Read voltage calibration based on host io operations

ABSTRACT

Devices and techniques for read voltage calibration of a flash-based storage system based on host IO operations are disclosed. In an example, a memory device includes a NAND memory array having groups of multiple blocks of memory cells, and a memory controller to optimize voltage calibration for reads of the memory array. In an example, the optimization technique includes monitoring read operations occurring to a respective block, identifying a condition to trigger a read level calibration based on the read operations, and performing the read level calibration for the respective block or a memory component that includes the respective block. In a further example, the calibration is performed based on a threshold voltage to read the respective block, which may be considered when the threshold voltage to read the respective block is evaluated within a sampling operation performed by the read level calibration.

PRIORITY APPLICATION

This application is a continuation of U.S. application Ser. No.16/782,720, filed Feb. 5, 2020, which is a divisional of U.S.application Ser. No. 16/436,567, filed Jun. 10, 2019, now issued as U.S.Pat. No. 10,586,602, which is a divisional of U.S. application Ser. No.15/689,747, filed Aug. 29, 2017 and issued as U.S. Pat. No. 10,347,344,each of which is incorporated herein by reference in its entirety.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory, including volatile and non-volatilememory.

Volatile memory requires power to maintain its data, and includesrandom-access memory (RAM), dynamic random-access memory (DRAM), orsynchronous dynamic random-access memory (SDRAM), among others.

Non-volatile memory can retain stored data when not powered, andincludes flash memory, read-only memory (ROM), electrically erasableprogrammable ROM (EEPROM), static RAM (SRAM), erasable programmable ROM(EPROM), resistance variable memory, such as phase-change random-accessmemory (PCRAM), resistive random-access memory (RRAM), magnetoresistiverandom-access memory (MRAM), or 3D XPoint™ memory, among others.

Flash memory is utilized as non-volatile memory for a wide range ofelectronic applications. Flash memory devices typically include one ormore groups of one-transistor, floating gate or charge trap memory cellsthat allow for high memory densities, high reliability, and low powerconsumption.

Two common types of flash memory array architectures include NAND andNOR architectures, named after the logic form in which the basic memorycell configuration of each is arranged. The memory cells of the memoryarray are typically arranged in a matrix. In an example, the gates ofeach floating gate memory cell in a row of the array are coupled to anaccess line (e.g., a word line). In a NOR architecture, the drains ofeach memory cell in a column of the array are coupled to a data line(e.g., a bit line). In a NAND architecture, the drains of each memorycell in a string of the array are coupled together in series, source todrain, between a source line and a bit line.

Both NOR and NAND architecture semiconductor memory arrays are accessedthrough decoders that activate specific memory cells by selecting theword line coupled to their gates. In a NOR architecture semiconductormemory array, once activated, the selected memory cells place their datavalues on bit lines, causing different currents to flow depending on thestate at which a particular cell is programmed. In a NAND architecturesemiconductor memory array, a high bias voltage is applied to adrain-side select gate (SGD) line. Word lines coupled to the gates ofthe unselected memory cells of each group are driven at a specified passvoltage (e.g., Vpass) to operate the unselected memory cells of eachgroup as pass transistors (e.g., to pass current in a manner that isunrestricted by their stored data values). Current then flows from thesource line to the bit line through each series coupled group,restricted only by the selected memory cells of each group, placingcurrent encoded data values of selected memory cells on the bit lines.

Each flash memory cell in a NOR or NAND architecture semiconductormemory array can be programmed individually or collectively to one or anumber of programmed states. For example, a single-level cell (SLC) canrepresent one of two programmed states (e.g., 1 or 0), representing onebit of data.

However, flash memory cells can also represent one of more than twoprogrammed states, allowing the manufacture of higher density memorieswithout increasing the number of memory cells, as each cell canrepresent more than one binary digit (e.g., more than one bit). Suchcells can be referred to as multi-state memory cells, multi-digit cells,or multi-level cells (MLCs). In certain examples, MLC can refer to amemory cell that can store two bits of data per cell (e.g., one of fourprogrammed states), a triple-level cell (TLC) can refer to a memory cellthat can store three bits of data per cell (e.g., one of eightprogrammed states), and a quad-level cell (QLC) can store four bits ofdata per cell. MLC is used herein in its broader context, to can referto any memory cell that can store more than one bit of data per cell(i.e., that can represent more than two programmed states).

Traditional memory arrays are two-dimensional (2D) structures arrangedon a surface of a semiconductor substrate. To increase memory capacityfor a given area, and to decrease cost, the size of the individualmemory cells has decreased. However, there is a technological limit tothe reduction in size of the individual memory cells, and thus, to thememory density of 2D memory arrays. In response, three-dimensional (3D)memory structures, such as 3D NAND architecture semiconductor memorydevices, are being developed to further increase memory density andlower memory cost.

Such 3D NAND devices often include strings of storage cells, coupled inseries (e.g., drain to source), between one or more source-side selectgates (SGSs) proximate a source, and one or more drain-side select gates(SGDs) proximate a bit line. In an example, the SGSs or the SGDs caninclude one or more field-effect transistors (FETs) or metal-oxidesemiconductor (MOS) structure devices, etc. In some examples, thestrings will extend vertically, through multiple vertically spaced tierscontaining respective word lines. A semiconductor structure (e.g., apolysilicon structure) may extend adjacent a string of storage cells toform a channel for the storages cells of the string. In the example of avertical string, the polysilicon structure may be in the form of avertically extending pillar. In some examples the string may be“folded,” and thus arranged relative to a U-shaped pillar. In otherexamples, multiple vertical structures may be stacked upon one anotherto form stacked arrays of storage cell strings.

Memory arrays or devices can be combined together to form a storagevolume of a memory system, such as a solid-state drive (SSD), aUniversal Flash Storage (UFS™) device, a MultiMediaCard (MMC)solid-state storage device, an embedded MMC device (eMMC™), etc. An SSDcan be used as, among other things, the main storage device of acomputer, having advantages over traditional hard drives with movingparts with respect to, for example, performance, size, weight,ruggedness, operating temperature range, and power consumption. Forexample, SSDs can have reduced seek time, latency, or other delayassociated with magnetic disk drives (e.g., electromechanical, etc.).SSDs use non-volatile memory cells, such as flash memory cells toobviate internal battery supply requirements, thus allowing the drive tobe more versatile and compact.

An SSD can include a number of memory devices, including a number ofdies or logical units (e.g., logical unit numbers or LUNs), and caninclude one or more processors or other controllers performing logicfunctions required to operate the memory devices or interface withexternal systems. Such SSDs may include one or more flash memory die,including a number of memory arrays and peripheral circuitry thereon.The flash memory arrays can include a number of blocks of memory cellsorganized into a number of physical pages. In many examples, the SSDswill also include DRAM or SRAM (or other forms of memory die or othermemory structures). The SSD can receive commands from a host inassociation with memory operations, such as read or write operations totransfer data (e.g., user data and associated integrity data, such aserror data and address data, etc.) between the memory devices and thehost, or erase operations to erase data from the memory devices.

In NAND flash based storage systems, the read voltage threshold (Vt)that is required to successfully perform read operations is constantlysubjected to shifts. These shifts may occur due to well-known stresseson the NAND flash such as Read Disturb, Data Retention,Cross-temperature effect, among other conditions. Further, differentNAND blocks within a memory array may experience a varying amount ofstress that induces a varying amount of charge loss or charge gain;likewise, different NAND blocks of an array are often written and readat different temperatures. As a result, a mismatch between the NAND Vtand the read voltage actually used by a storage system may occur in manyscenarios. Various techniques for read voltage calibration are used bymany NAND storage systems to adjust the read voltage in accordance withNAND Vt. However, existing approaches for launching and utilizing readvoltage calibration often fail to fully address the voltage thresholdshift that has occurred in a particular area of memory being read,leading to unwanted data errors from read operations, additionalcalibration and adjustment operations being requested and performed, anddelays and reduced performance of the storage system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 illustrates an example of an environment including a memorydevice.

FIGS. 2-3 illustrate schematic diagrams of an example of a 3D NANDarchitecture semiconductor memory array.

FIG. 4 illustrates an example block diagram of a memory module.

FIG. 5 illustrates a block diagram of an example system including amemory device adapted for implementing an optimized technique for readvoltage calibration.

FIG. 6 illustrates an operation sequence diagram of an example memorydevice configuration adapted for performing an optimized technique forread voltage calibration.

FIG. 7 illustrates a flowchart of an example set of operations adaptedfor performing an optimized technique for read voltage calibration.

FIG. 8 is a block diagram illustrating an example of a machine uponwhich one or more embodiments may be implemented.

DETAILED DESCRIPTION

The systems, processes, and configurations discussed herein relate tooptimization techniques for triggering and performing read voltagecalibration of a NAND memory device. Specifically, example techniquesare disclosed that utilize the physical footprint of host read IO, inreal time, for selecting blocks or other areas of a memory array to besampled with a voltage level calibration. The calibration of the readvoltages may be based on the most frequently read data locations, thusreducing trigger rates and calibrating Vt movement for targetedbenchmark stress conditions. Further, example techniques are disclosedthat enable intelligent and adaptive block sampling criterion whichfavors active areas of a memory array (e.g., respective blocks andgroups of blocks most frequently accessed by host IO), to calibrate NANDread voltages for best possible read performance in ongoing and futureoperations. Still further, example techniques are disclosed thatidentify conditions to trigger the read voltage calibration and thattrack the state of areas of a memory array involved in host IOoperations (including read and erase activity for respective blocks andgroups of blocks).

Electronic devices, such as mobile electronic devices (e.g., smartphones, tablets, etc.), electronic devices for use in automotiveapplications (e.g., automotive sensors, control units, driver-assistancesystems, passenger safety or comfort systems, etc.), andinternet-connected appliances or devices (e.g., internet-of-things (IoT)devices, etc.), have varying storage needs depending on, among otherthings, the type of electronic device, use environment, performanceexpectations, etc.

Electronic devices can be broken down into several main components: aprocessor (e.g., a central processing unit (CPU) or other mainprocessor); memory (e.g., one or more volatile or non-volatilerandom-access memory (RAM) memory device, such as dynamic RAM (DRAM),mobile or low-power double-data-rate synchronous DRAM (DDR SDRAM).etc.); and a storage device (e.g., non-volatile memory (NVM) device,such as flash memory, read-only memory (ROM), an SSD, an MMC, or othermemory card structure or assembly, etc.). In certain examples,electronic devices can include a user interface (e.g., a display,touch-screen, keyboard, one or more buttons, etc.), a graphicsprocessing unit (GPU), a power management circuit, a baseband processoror one or more transceiver circuits, etc.

FIG. 1 illustrates an example of an environment 100 including a hostdevice 105 and a memory device 110 configured to communicate over acommunication interface. The host device 105 or the memory device 110may be included in a variety of products 150, such as Internet of Things(IoT) devices (e.g., a refrigerator or other appliance, sensor, motor oractuator, mobile communication device, automobile, drone, etc.) tosupport processing, communications, or control of the product 150.

The memory device 110 includes a memory controller 115 and a memoryarray 120 including, for example, a number of individual memory die(e.g., a stack of three-dimensional (3D) NAND die). In 3D architecturesemiconductor memory technology, vertical structures are stacked,increasing the number of tiers, physical pages, and accordingly, thedensity of a memory device (e.g., a storage device). In an example, thememory device 110 can be a discrete memory or storage device componentof the host device 105. In other examples, the memory device 110 can bea portion of an integrated circuit (e.g., system on a chip (SOC), etc.),stacked or otherwise included with one or more other components of thehost device 105.

One or more communication interfaces can be used to transfer databetween the memory device 110 and one or more other components of thehost device 105, such as a Serial Advanced Technology Attachment (SATA)interface, a Peripheral Component Interconnect Express (PCIe) interface,a Universal Serial Bus (USB) interface, a Universal Flash Storage (UFS)interface, an eMMCT™ interface, or one or more other connectors orinterfaces. The host device 105 can include a host system, an electronicdevice, a processor, a memory card reader, or one or more otherelectronic devices external to the memory device 110. In some examples,the host 105 may be a machine having some portion, or all, of thecomponents discussed in reference to the machine 800 of FIG. 8.

The memory controller 115 can receive instructions from the host 105,and can communicate with the memory array, such as to transfer data to(e.g., write or erase) or from (e.g., read) one or more of the memorycells, planes, sub-blocks, blocks, or pages of the memory array. Thememory controller 115 can include, among other things, circuitry orfirmware, including one or more components or integrated circuits. Forexample, the memory controller 115 can include one or more memorycontrol units, circuits, or components configured to control accessacross the memory array 120 and to provide a translation layer betweenthe host 105 and the memory device 110. The memory controller 115 caninclude one or more input/output (I/O) circuits, lines, or interfaces totransfer data to or from the memory array 120. The memory controller 115can include a memory manager 125 and an array controller 135.

The memory manager 125 can include, among other things, circuitry orfirmware, such as a number of components or integrated circuitsassociated with various memory management functions. For purposes of thepresent description example memory operation and management functionswill be described in the context of NAND memory. Persons skilled in theart will recognize that other forms of non-volatile memory may haveanalogous memory operations or management functions. Such NANDmanagement functions include wear leveling (e.g., garbage collection orreclamation), error detection or correction, block retirement, or one ormore other memory management functions. The memory manager 125 can parseor format host commands (e.g., commands received from a host) intodevice commands (e.g., commands associated with operation of a memoryarray, etc.), or generate device commands (e.g., to accomplish variousmemory management functions) for the array controller 135 or one or moreother components of the memory device 110.

The memory manager 125 can include a set of management tables 130configured to maintain various information associated with one or morecomponent of the memory device 110 (e.g., various information associatedwith a memory array or one or more memory cells coupled to the memorycontroller 115). For example, the management tables 130 can includeinformation regarding block age, block erase count, error history, orone or more error counts (e.g., a write operation error count, a readbit error count, a read operation error count, an erase error count,etc.) for one or more blocks of memory cells coupled to the memorycontroller 115. In certain examples, if the number of detected errorsfor one or more of the error counts is above a threshold, the bit errorcan be referred to as an uncorrectable bit error. The management tables130 can maintain a count of correctable or uncorrectable bit errors,among other things.

The array controller 135 can include, among other things, circuitry orcomponents configured to control memory operations associated withwriting data to, reading data from, or erasing one or more memory cellsof the memory device 110 coupled to the memory controller 115. Thememory operations can be based on, for example, host commands receivedfrom the host 105, or internally generated by the memory manager 125(e.g., in association with wear leveling, error detection or correction,etc.).

The array controller 135 can include an error correction code (ECC)component 140, which can include, among other things, an ECC engine orother circuitry configured to detect or correct errors associated withwriting data to or reading data from one or more memory cells of thememory device 110 coupled to the memory controller 115. The memorycontroller 115 can be configured to actively detect and recover fromerror occurrences (e.g., bit errors, operation errors, etc.) associatedwith various operations or storage of data, while maintaining integrityof the data transferred between the host 105 and the memory device 110,or maintaining integrity of stored data (e.g., using redundant RAIDstorage, etc.), and can remove (e.g., retire) failing memory resources(e.g., memory cells, memory arrays, pages, blocks, etc.) to preventfuture errors.

The memory array 120 can include several memory cells arranged in, forexample, a number of devices, planes, sub-blocks, blocks, or pages. Asone example, a 48 GB TLC NAND memory device can include 18,592 bytes (B)of data per page (16,384+2208 bytes), 1536 pages per block, 548 blocksper plane, and 4 or more planes per device. As another example, a 32 GBMLC memory device (storing two bits of data per cell (i.e., 4programmable states)) can include 18,592 bytes (B) of data per page(16.384+2208 bytes), 1024 pages per block, 548 blocks per plane, and 4planes per device, but with half the required write time and twice theprogram/erase (P/E) cycles as a corresponding TLC memory device. Otherexamples can include other numbers or arrangements. In some examples, amemory device, or a portion thereof, may be selectively operated in SLCmode, or in a desired MLC mode (such as TLC, QLC, etc.).

In operation, data is typically written to or read from the NAND memorydevice 110 in pages, and erased in blocks. However, one or more memoryoperations (e.g., read, write, erase, etc.) can be performed on largeror smaller groups of memory cells, as desired. The data transfer size ofa NAND memory device 110 is typically referred to as a page, whereas thedata transfer size of a host is typically referred to as a sector.

Although a page of data can include a number of bytes of user data(e.g., a data payload including a number of sectors of data) and itscorresponding metadata, the size of the page often refers only to thenumber of bytes used to store the user data. As an example, a page ofdata having a page size of 4 KB may include 4 KB of user data (e.g., 8sectors assuming a sector size of 512 B) as well as a number of bytes(e.g., 32 B, 54 B, 224 B, etc.) of metadata corresponding to the userdata, such as integrity data (e.g., error detecting or correcting codedata), address data (e.g., logical address data, etc.), or othermetadata associated with the user data.

Different types of memory cells or memory arrays 120 can provide fordifferent page sizes, or may require different amounts of metadataassociated therewith. For example, different memory device types mayhave different bit error rates, which can lead to different amounts ofmetadata necessary to ensure integrity of the page of data (e.g., amemory device with a higher bit error rate may require more bytes oferror correction code data than a memory device with a lower bit errorrate). As an example, a multi-level cell (MLC) NAND flash device mayhave a higher bit error rate than a corresponding single-level cell(SLC) NAND flash device. As such, the MLC device may require moremetadata bytes for error data than the corresponding SLC device.

FIG. 2 illustrates an example schematic diagram of a 3D NANDarchitecture semiconductor memory array 200 including a number ofstrings of memory cells (e.g., first-third A₀ memory strings205A₀-207A₀, first-third A_(n) memory strings 205A_(n)-207A_(n),first-third B₀ memory strings 205B₀-207B₀, first-third B_(n) memorystrings 205B_(n)-207B_(n), etc.), organized in blocks (e.g., block A201A, block B 201B, etc.) and sub-blocks (e.g., sub-block A₀ 201A₀,sub-block A_(n) 201A_(n), sub-block B₀ 201B₀, sub-block B_(n) 201B_(n),etc.). The memory array 200 represents a portion of a greater number ofsimilar structures that would typically be found in a block, device, orother unit of a memory device.

Each string of memory cells includes a number of tiers of charge storagetransistors (e.g., floating gate transistors, charge-trappingstructures, etc.) stacked in the Z direction, source to drain, between asource line (SRC) 235 or a source-side select gate (SGS) (e.g.,first-third A₀ SGS 231A₀-233A₀, first-third A_(n) SGS 231A_(n)-233A_(n),first-third B₀ SGS 231B₀-233B₀, first-third B_(n) SGS 231B_(n)-233B_(n),etc.) and a drain-side select gate (SGD) (e.g., first-third A₀ SGD226A₀-228A₀, first-third A_(n) SGD 226A_(n)-228A_(n), first-third B₀ SGD226B₀-228B₀, first-third B_(n) SGD 226B_(n)-228B_(n), etc.). Each stringof memory cells in the 3D memory array can be arranged along the Xdirection as data lines (e.g., bit lines (BL) BL0-BL2 220-222), andalong the Y direction as physical pages.

Within a physical page, each tier represents a row of memory cells, andeach string of memory cells represents a column. A sub-block can includeone or more physical pages. A block can include a number of sub-blocks(or physical pages) (e.g., 128, 256, 384, etc.). Although illustratedherein as having two blocks, each block having two sub-blocks, eachsub-block having a single physical page, each physical page having threestrings of memory cells, and each string having 8 tiers of memory cells,in other examples, the memory array 200 can include more or fewerblocks, sub-blocks, physical pages, strings of memory cells, memorycells, or tiers. For example, each string of memory cells can includemore or fewer tiers (e.g., 16, 32, 64, 128, etc.), as well as one ormore additional tiers of semiconductor material above or below thecharge storage transistors (e.g., select gates, data lines, etc.), asdesired. As an example, a 48 GB TLC NAND memory device can include18,592 bytes (B) of data per page (16.384+2208 bytes), 1536 pages perblock, 548 blocks per plane, and 4 or more planes per device.

Each memory cell in the memory array 200 includes a control gate (CG)coupled to (e.g., electrically or otherwise operatively connected to) anaccess line (e.g., word lines (WL) WL0 ₀-WL7 ₀ 210A-217A, WL0 ₁-WL7 ₁210B-217B, etc.), which collectively couples the control gates (CGs)across a specific tier, or a portion of a tier, as desired. Specifictiers in the 3D memory array, and accordingly, specific memory cells ina string, can be accessed or controlled using respective access lines.Groups of select gates can be accessed using various select lines. Forexample, first-third A₀ SGD 226A₀-228A₀ can be accessed using an A₀ SGDline SGDA₀ 225A₀, first-third A_(n) SGD 226A_(n)-228A_(n) can beaccessed using an A_(n) SGD line SGDA_(n) 225A_(n), first-third B₀ SGD226B₀-228B₀ can be accessed using an B₀ SGD line SGDB₀ 225B₀, andfirst-third B_(n) SGD 226B_(n)-228B_(n) can be accessed using an B_(n)SGD line SGDB_(n) 225B_(n). First-third A₀ SGS 231A₀-233A₀ andfirst-third A_(n) SGS 231A_(n)-233A_(n) can be accessed using a gateselect line SGS₀ 230A, and first-third B₀ SGS 231B₀-233B₀ andfirst-third B_(n) SGS 231B_(n)-233B_(n) can be accessed using a gateselect line SGS₁ 230B.

In an example, the memory array 200 can include a number of levels ofsemiconductor material (e.g., polysilicon, etc.) configured to couplethe control gates (COs) of each memory cell or select gate (or a portionof the CGs or select gates) of a respective tier of the array. Specificstrings of memory cells in the array can be accessed, selected, orcontrolled using a combination of bit lines (BLs) and select gates,etc., and specific memory cells at one or more tiers in the specificstrings can be accessed, selected, or controlled using one or moreaccess lines (e.g., word lines).

FIG. 3 illustrates an example schematic diagram of a portion of a NANDarchitecture semiconductor memory array 300 including a plurality ofmemory cells 302 arranged in a two-dimensional array of strings (e.g.,first-third strings 305-307) and tiers (e.g., illustrated as respectiveword lines (WL) WL0-WL7 310-317, a drain-side select gate (SGD) line325, a source-side select gate (SGS) line 330, etc.), and senseamplifiers or devices 360. For example, the memory array 300 canillustrate an example schematic diagram of a portion of one physicalpage of memory cells of a 3D NAND architecture semiconductor memorydevice, such as illustrated in FIG. 2.

Each string of memory cells is coupled to a source line (SRC) using arespective source-side select gate (SGS) (e.g., first-third SGS331-333), and to a respective data line (e.g., first-third bit lines(BL) BL0-BL2 320-322) using a respective drain-side select gate (SGD)(e.g., first-third SGD 326-328). Although illustrated with 8 tiers(e.g., using word lines (WL) WL0-WL7 310-317) and three data lines(BL0-BL2 326-328) in the example of FIG. 3, other examples can includestrings of memory cells having more or fewer tiers or data lines, asdesired.

In a NAND architecture semiconductor memory array, such as the examplememory array 300, the state of a selected memory cell 302 can beaccessed by sensing a current or voltage variation associated with aparticular data line containing the selected memory cell. The memoryarray 300 can be accessed (e.g., by a control circuit, one or moreprocessors, digital logic, etc.) using one or more drivers. In anexample, one or more drivers can activate a specific memory cell, or setof memory cells, by driving a particular potential to one or more datalines (e.g., bit lines BL0-BL2), access lines (e.g., word linesWL0-WL7), or select gates, depending on the type of operation desired tobe performed on the specific memory cell or set of memory cells.

To program or write data to a memory cell, a programming voltage (Vpgm)(e.g., one or more programming pulses, etc.) can be applied to selectedword lines (e.g., WL4), and thus, to a control gate of each memory cellcoupled to the selected word lines (e.g., first-third control gates(CGs) 341-343 of the memory cells coupled to WL4). Programming pulsescan begin, for example, at or near 15V, and, in certain examples, canincrease in magnitude during each programming pulse application. Whilethe program voltage is applied to the selected word lines, a potential,such as a ground potential (e.g., Vss), can be applied to the data lines(e.g., bit lines) and substrates (and thus the channels, between thesources and drains) of the memory cells targeted for programming,resulting in a charge transfer (e.g., direct injection orFowler-Nordheim (FN) tunneling, etc.) from the channels to the floatinggates of the targeted memory cells.

In contrast, a pass voltage (Vpass) can be applied to one or more wordlines having memory cells that are not targeted for programming, or aninhibit voltage (e.g., Vcc) can be applied to data lines (e.g., bitlines) having memory cells that are not targeted for programming, forexample, to inhibit charge from being transferred from the channels tothe floating gates of such non-targeted memory cells. The pass voltagecan be variable, depending, for example, on the proximity of the appliedpass voltages to a word line targeted for programming. The inhibitvoltage can include a supply voltage (Vcc), such as a voltage from anexternal source or supply (e.g., a battery, an AC-to-DC converter,etc.), relative to a ground potential (e.g., Vss).

As an example, if a programming voltage (e.g., 15V or more) is appliedto a specific word line, such as WL4, a pass voltage of 10V can beapplied to one or more other word lines, such as WL3, WL5, etc., toinhibit programming of non-targeted memory cells, or to retain thevalues stored on such memory cells not targeted for programming. As thedistance between an applied program voltage and the non-targeted memorycells increases, the pass voltage required to refrain from programmingthe non-targeted memory cells can decrease. For example, where aprogramming voltage of 15V is applied to WL4, a pass voltage of 10V canbe applied to WL3 and WL5, a pass voltage of 8V can be applied to WL2and WL6, a pass voltage of 7V can be applied to WL1 and WL7, etc. Inother examples, the pass voltages, or number of word lines, etc., can behigher or lower, or more or less.

The sense amplifiers 360, coupled to one or more of the data lines(e.g., first, second, or third bit lines (BL0-BL2) 320-322), can detectthe state of each memory cell in respective data lines by sensing avoltage or current on a particular data line.

Between applications of one or more programming pulses (e.g., Vpgm), averify operation can be performed to determine if a selected memory cellhas reached its intended programmed state. If the selected memory cellhas reached its intended programmed state, it can be inhibited fromfurther programming. If the selected memory cell has not reached itsintended programmed state, additional programming pulses can be applied.If the selected memory cell has not reached its intended programmedstate after a particular number of programming pulses (e.g., a maximumnumber), the selected memory cell, or a string, block, or pageassociated with such selected memory cell, can be marked as defective.

To erase a memory cell or a group of memory cells (e.g., erasure istypically performed in blocks or sub-blocks), an erasure voltage (Vers)(e.g., typically Vpgm) can be applied to the substrates (and thus thechannels, between the sources and drains) of the memory cells targetedfor erasure (e.g., using one or more bit lines, select gates, etc.),while the word lines of the targeted memory cells are kept at apotential, such as a ground potential (e.g., Vss), resulting in a chargetransfer (e.g., direct injection or Fowler-Nordheim (FN) tunneling,etc.) from the floating gates of the targeted memory cells to thechannels.

FIG. 4 illustrates an example block diagram of a memory device 400including a memory array 402 having a plurality of memory cells 404, andone or more circuits or components to provide communication with, orperform one or more memory operations on, the memory array 402. Thememory device 400 can include a row decoder 412, a column decoder 414,sense amplifiers 420, a page buffer 422, a selector 424, an input/output(I/O) circuit 426, and a memory control unit 430.

The memory cells 404 of the memory array 402 can be arranged in blocks,such as first and second blocks 402A, 402B. Each block can includesub-blocks. For example, the first block 402A can include first andsecond sub-blocks 402A₀, 402A_(n), and the second block 402B can includefirst and second sub-blocks 402B₀, 402B_(n). Each sub-block can includea number of physical pages, each page including a number of memory cells404. Although illustrated herein as having two blocks, each block havingtwo sub-blocks, and each sub-block having a number of memory cells 404,in other examples, the memory array 402 can include more or fewerblocks, sub-blocks, memory cells, etc. In other examples, the memorycells 404 can be arranged in a number of rows, columns, pages,sub-blocks, blocks, etc., and accessed using, for example, access lines406, first data lines 410, or one or more select gates, source lines,etc.

The memory control unit 430 can control memory operations of the memorydevice 400 according to one or more signals or instructions received oncontrol lines 432, including, for example, one or more clock signals orcontrol signals that indicate a desired operation (e.g., write, read,erase, etc.), or address signals (A0-AX) received on one or more addresslines 416. One or more devices external to the memory device 400 cancontrol the values of the control signals on the control lines 432, orthe address signals on the address line 416. Examples of devicesexternal to the memory device 400 can include, but are not limited to, ahost, a memory controller, a processor, or one or more circuits orcomponents not illustrated in FIG. 4.

The memory device 400 can use access lines 406 and first data lines 410to transfer data to (e.g., write or erase) or from (e.g., read) one ormore of the memory cells 404. The row decoder 412 and the column decoder414 can receive and decode the address signals (A0-AX) from the addressline 416, can determine which of the memory cells 404 are to beaccessed, and can provide signals to one or more of the access lines 406(e.g., one or more of a plurality of word lines (WL0-WLm)) or the firstdata lines 410 (e.g., one or more of a plurality of bit lines(BL0-BLn)), such as described above.

The memory device 400 can include sense circuitry, such as the senseamplifiers 420, configured to determine the values of data on (e.g.,read), or to determine the values of data to be written to, the memorycells 404 using the first data lines 410. For example, in a selectedstring of memory cells 404, one or more of the sense amplifiers 420 canread a logic level in the selected memory cell 404 in response to a readcurrent flowing in the memory array 402 through the selected string tothe data lines 410.

One or more devices external to the memory device 400 can communicatewith the memory device 400 using the I/O lines (DQ0-DQN) 408, addresslines 416 (A0-AX), or control lines 432. The input/output (I/O) circuit426 can transfer values of data in or out of the memory device 400, suchas in or out of the page buffer 422 or the memory array 402, using theI/O lines 408, according to, for example, the control lines 432 andaddress lines 416. The page buffer 422 can store data received from theone or more devices external to the memory device 400 before the data isprogrammed into relevant portions of the memory array 402, or can storedata read from the memory array 402 before the data is transmitted tothe one or more devices external to the memory device 400.

The column decoder 414 can receive and decode address signals (A0-AX)into one or more column select signals (CSEL1-CSELn). The selector 424(e.g., a select circuit) can receive the column select signals(CSEL1-CSELn) and select data in the page buffer 422 representing valuesof data to be read from or to be programmed into memory cells 404.Selected data can be transferred between the page buffer 422 and the I/Ocircuit 426 using second data lines 418.

The memory control unit 430 can receive positive and negative supplysignals, such as a supply voltage (Vcc) 434 and a negative supply (Vss)436 (e.g., a ground potential), from an external source or supply (e.g.,an internal or external battery, an AC-to-DC converter, etc.). Incertain examples, the memory control unit 430 can include a regulator428 to internally provide positive or negative supply signals.

The following techniques and configurations discussed herein providetechniques implemented within a NAND memory device for aspects of readvoltage calibration. The techniques and configurations discussed hereinmay be specifically applicable to a 3D NAND flash based storage system,such as a system embodying the 3D NAND architecture features discussedabove. However, it will be apparent that the disclosed read voltagecalibration techniques and configurations may also apply to other formsof NAND flash devices, including with components of NAND flash devicesapplied in other form factors or arrangements.

In NAND flash based storage systems, a NAND memory array includesvarious blocks that experience different amount of charge loss andcharge gain, and various blocks that are programmed (written) atdifferent temperatures. Thus, the Vt to read any particular block orarea of the memory array may vary from another block or another area ofthe memory array. Further, the Vt for a particular block or area ofmemory is constantly subjected to shifts due to stresses, and as aresult, Vt may shift in either direction.

Read disturb is an example of a stress occurring on a NAND flash devicethat affects Vt. As a host or application retrieves certain data fromthe flash device, the read disturb stress may build up if the hostutilizes a particularly high read rate or read intensive application ofdata (depending on how the data is scattered within the flash device).For instance, if a logical block address (LBA) maps to a particularphysical location of a NAND block, because of the biasing conditionwithin the block, stresses may be induced on the unselected word linesand memory cells may become biased from one another.

Cross-temperature effect is another example of a stress occurring on aNAND flash device that affects Vt. Cross-temperature effect causes ashift in the cell Vt in the NAND blocks, with positive or negative Vtshifts, as a result of a temperature difference between the time ofwriting and the time of reading. For instance, as data is written to aNAND flash device (e.g., a SSD drive, or SD/MMC card), the Vt needed toread the data is based on the ambient temperature when the data waswritten. However, because the data may remain resident on the flashdevice for long period of time, the flash device is unlikely to be readat the same temperature.

A mismatch between a read voltage used to read a block and the Vt for aparticular block will result in data errors. One representation of dataerrors, the fail bit count, may be measured in a NAND storage system asa “raw bit error rate” (RBER). As a result, RBER provides a function ofthe mismatch between the read voltage and the Vt. Thus, in the operationof many existing NAND flash-based storage systems, the RBER provides ameasurement to determine whether read voltage is incorrect, and whethervoltage calibration should be conducted.

The RBER can be minimized by adjusting the read voltage in accordancewith Vt, through the use of read voltage calibration. Read voltagecalibration may be initiated using a reactive or a proactive method.With a reactive method, read voltage calibration is performed inresponse to a read error that occurred during the host read 10. Thereactive method uses a targeted criteria to calibrate the read voltagedirectly in response to the read error. In contrast, a proactive methodfor read voltage calibration uses a sampling approach to periodicallytrack average Vt shifts, and launch calibration before significanterrors occur. Existing approaches for proactively performing read levelcalibration involves sampling of the RBER at different read voltages,and choosing the most optimal read voltage to optimize (e.g., reduce)RBER. For instance, storage device firmware may calibrate NAND readvoltage by scanning multiple pages among various locations in a NANDmemory array, performing reads at the multiple pages at various readvoltages, measuring the resulting RBER, and choosing one of the readvoltages with a minimal error rate.

Thus, a primary objective of read voltage calibration is to minimize theerror handling trigger rate for the system that will occur during hostread 10. With proactive read voltage calibration processes, a variety ofblocks of the memory device are sampled to determine average thresholdvoltage, and to periodically track the average Vt shifts for variousblocks of the memory device. The block sampling criteria used in a readvoltage calibration (e.g., by sampling a majority of blocks in a flashdevice) affects the accuracy of a resulting read voltage setting. Forinstance, many read voltage calibration techniques utilize randomsampling of blocks or data age-based sampling, which tends correlate thevoltage calibration to the majority state of the flash device. However,the majority state of the flash device does not necessarily correspondto the majority of the blocks accessed by the host read IO at aparticular time on the flash device. Thus, existing approaches formajority state-based sampling criteria for proactive read voltagecalibration may not be fully effective in reducing an error handlingtrigger rate and RBER.

A simple explanation of why sampling may not be effective, is becausethe Vt corresponding to locations of host read IO may not necessarilyalign with a voltage setting determined from sampling a majority stateof the storage device. For example, consider a scenario where an SSDdrive stores archived media files, rarely accessed by the host, whichoccupy up to 70% of the total drive space. The host, however, accessesother files on remaining 30% of the drive space relatively frequently.In this scenario, calibrating read voltages based on random sampling ofblocks would not be optimal, because the resulting calibration wouldtend to over-compensate for the existence of the archived media files.Thus, calibrating read voltages based on sampling the majority state ofthe drive may not accurately adjust threshold voltage—and hence may notreduce the error handling triggers—associated with the host IO and themost frequently accessed portion of the host data.

The techniques discussed herein include use of the physical footprint ofhost read IO in real time for selecting sample blocks for read voltagecalibration. In host IO-based sampling, a controller of a memory devicetracks the host IO footprint at a page, word line, or physical blocklevel, and uses this tracked information for identifying read voltagecalibration block sampling candidates. The controller calibrates theread voltages based on the most frequently read data, thus reducingtrigger rates. In an example, this tracking technique may involvetracking read locations and read frequency used in host IO operationsfor subsequent sampling, and monitoring error rates occurring with suchlocations. Also in an example, this tracking technique may involvetracking a number of reads or erases to a particular unit of the memory(e.g., a page, a block, a die, etc.).

By using the physical footprint of host IO, a memory device may operateto more effectively detect and calibrate NAND Vt movement for readintensive applications as well as targeted benchmark stress conditions.Further, the memory device may utilize a smarter block samplingcriterion to favor blocks or other physical units of memory that aremost often accessed by host IO to calibrate NAND read voltages for bestpossible read performance.

The techniques discussed herein thus improve the error handling triggerrates by aligning NAND Vt shifts with the host read IO footprint at aphysical level. The techniques also achieve improved readperformance/latency for targeted benchmarks versus existing calibrationapproaches, with minimal or zero impact on normal user workloads.Additionally, the techniques helps alleviate NAND trigger raterequirements due to targeted benchmark and user workloads, which helpseliminate NAND over-design that might otherwise result in NAND enduranceor performance penalty.

FIG. 5 provides a block diagram of an example system 500 including amemory device 510 (e.g., a SSD storage device, a SD/MMC card, etc.)adapted for implementing the read voltage calibration optimizationdiscussed herein. As shown, the memory device 510 includes a NAND memoryarray 530 with multiple dies (dies 1-N), with each die including one ormore blocks (blocks 1-N). Each of the one or more blocks may includefurther divided portions, such as one or more pages (not shown) perblock; each of the one or more pages may be further divided into one ormore word lines (not shown) per page; each of the word lines may bedivided into one or more memory cells (not shown).

In an example, the blocks of memory cells of the memory array 530include groups of at least one of: single-level cell (SLC), multi-layercell (MLC), triple-layer cell (TLC), or quad-layer cell (QLC) NANDmemory cells. Also, in an example, the memory array 530 is arranged intoa stack of three-dimensional (3D) NAND dies, such that the respectivegroup of multiple blocks hosting a respective block is a member of agroup of blocks provided by a respective die in the stack of 3D NANDdies. These configurations and further detailed components of the memoryarray 530 are not illustrated in FIG. 5 for simplicity. However, thememory array 530 may incorporate these or any of the features describedabove with reference to features of 3D NAND architecture devices orother forms of NAND storage devices.

The memory device 510 is shown as being operably coupled to a host 520via a controller 540. The controller 540 is adapted to receive andprocess host commands 525, such as read operations, write operations,erase operations, and the like, to read, write, erase, and similaroperations within the memory array 530. A variety of other componentsfor the memory device 510 (such as a memory manager, and other circuitryor operational components) and the controller 540 are also not depictedfor simplicity.

The controller 540 is depicted as including a memory 544 (e.g., volatilememory), processing circuitry 546 (e.g., a microprocessor), and astorage media 548 (e.g., non-volatile memory), used for executinginstructions (e.g., instructions hosted by the storage media 548, loadedinto memory 544, and executed by the processing circuitry 546) toperform control operations 542 for management and use of the memoryarray 530. The control operations 542 performed or facilitated by thecontroller 540 may be provided from various types of hardware, firmware,and software functionality (e.g., logic implemented in hardware,specially programmed modules, etc.).

The functionality provided by the control operations 542 may include: IOoperation monitoring 550 (e.g., to monitor read and erase operations,originating from host commands); host operation processing 560 (e.g., tointerpret and process the host commands 525, and to issue furthercommands to the memory array 530 to perform respective read, write,erase, or other host-initiated operations); read voltage control 570(e.g., to establish, set, and utilize a read voltage level to read aparticular portion of the memory array 530); read level calibration 580(e.g., to operate a calibration procedure to identify a new read voltagelevel of a particular portion or portions of the memory array 530); anderror detection processing 590 (e.g., to identify and correct errorsfrom data obtained in read operations, to identify one or more RBER(s)for a particular read operation or set of operations, etc.).

In an example, the IO operation monitoring 550 operates to track readsand writes to the memory array, and also to track accompanying readoperations and write operations relevant to voltage level andcalibration. Further, the IO operation monitoring 550 may identifycharacteristics of various blocks and block operations within the memoryarray 530, and identify blocks subjected to frequent read or eraseoperations (e.g., blocks that are read more frequently than otherblocks, or blocks that have been erased more times than other blocks).As further detailed in the examples discussed in FIG. 6 below, this mayinclude the use of counters established for IO operations, establishedat a page, block, or die level, such as a read counter to measure thenumber of reads occurring to a respective block or die, or a erasecounter to measure the number of erases occurring to a respective block.These counters may be used as triggers for calibration, data inputs tosampling operations performed in calibration, as discussed in theexamples of FIG. 6 below.

In an example, the read voltage control 570 is used to establish,change, and provide a voltage value used to read a particular area ofmemory (such as a respective block in the memory array 530). Forexample, the read voltage control 570 may implement various positive ornegative offsets in order to read respective memory cells and memorylocations (e.g., pages, blocks, dies) including the respective memorycells.

In an example, the read level calibration 580 is used to establish(e.g., change, update, reset, etc.) the value of the read voltageimplemented by the read voltage control 570. The read level calibration580 may be implemented through multiple sampling commands 585 performedon the memory array 530, such as sampling commands issued at varyingvoltages to multiple areas of the memory array, which attempt todetermine a read voltage that is optimized to the Vt of those areas. Theread level calibration 580 may operate in connection with the featuresof the host operation processing 560 or the error detection processing590. For instance, the host operation processing 560 may identify memorylocations for sampling based on IO read operations to those locations;also for instance, the error detection processing 590 may trigger theread level calibration 580 in response to particular conditions oferrors or an error rate of read data as exceeding a particularthreshold.

In an example, the read level calibration 580 is performed on a per-diebasis, for all of the memory locations (e.g., blocks, pages, cells)within such a die. In another example, the read level calibration 580 isperformed for multiple areas of the memory array (e.g., multiple dies),through one or more calibration operations (e.g., a series ofcalibration operations). Also in an example, the error detectionprocessing 590 may detect a RBER, an unrecoverable bit error rate(UBER), or other measurements or error conditions for a memory cell, agroup of cells, or larger areas of the memory array (e.g., averages orsamples from a block, group of blocks, die, group of dies, etc.). Alsoin an example, the error detection processing 590 may operate to assistthe trigger of a calibration operation with the read level calibration580, or a tracking operation with the IO operation monitoring 550.

In an example, the data determined through the IO operation monitoring550 is used to change the characteristics of a voltage calibrationoperation performed by the read level calibration 580. For instance,specific memory locations being tracked from the 10 operation monitoring550 (e.g., tracking blocks being frequently read) may be utilized assampling locations within the read level calibration 580. Also forinstance, conditions determined from reads of the memory locations(e.g., respective blocks being repeatedly read, respective blocks beingread with high error rates, respective blocks being repeatedly erased)as tracked by the IO operation monitoring 550 may also provide atrigger, control, or other input to the read level calibration 580.

In a further example, the specific memory locations being tracked fromthe 10 operation monitoring 550 or the conditions of the memorylocations being tracked by the IO operation monitoring 550 may beutilized by the error detection processing 590 to determine a likelihoodof read voltage deviation from a mis-calibrated memory location. Thus,features of the IO operation monitoring 550, the host operationprocessing 560, and the error detection processing 590, may jointlyoperate to trigger a read level calibration operation, and to identifycharacteristics (such as sampling locations) of the read levelcalibration operation.

As discussed herein, the read level calibration 580 performs operationsbased on data patterns and data activity originating from a host IO readoperation footprint. This footprint takes advantage of the logical tophysical mapping of memory locations from a LBA address, since physicalmapping may be spread among contiguous, different, or even random blocksof memory. The technique discussed herein can track read operations notat a LBA address (logical) level, but rather at a memory cell, page, orblock (physical) level. Thus, voltage calibration can occur based onread operations to memory locations of a LBA address that span acrossmultiple blocks and dies.

Additionally, the techniques discussed herein may utilize the physicalfootprint of host IO, from conditional block sampling, to calibratesubsequent and ongoing read operations. This approach may be useful if acustomer intends to optimize read voltages for certain benchmarks, userpatterns, read access patterns. The techniques discussed herein mayprovide granular tracking of read voltage levels for respective memorylocations and chunks of data among multiple memory locations, thusallowing a calibration technique to focus efforts on relevant areas in amore efficient and targeted fashion.

In addition to the techniques discussed herein, a variety of existingcalibration approaches may be integrated and used in connection with theread level calibration 580. For instance, in addition to sampling thespecific memory locations involved IO operations, the read levelcalibration 580 may additionally obtain locations for sampling based on:random sampling of other blocks in the memory array, sampling of otherblocks in the memory array based on data age, or sampling of otherblocks in the memory array based on raw bit error rate (RBER)corresponding to the other blocks.

FIG. 6 illustrates an operation sequence diagram 600 of an examplememory device configuration adapted for performing an optimizedtechnique for read voltage calibration. As shown, the host 520 isoperably coupled to the NAND memory array 530 through the operation ofcontroller logic 630. In an example, the controller logic 630 implementscommands for management, control, and access of a subject memory, suchas by performing read and write commands to the NAND memory array 530.

As an example, read or write commands received from the host 520 areinterpreted by the controller logic 630 to identify memory locations inthe memory array for the read or write commands, which then result inrespective read and write operations on the identified memory locationsof the memory array 530. Likewise, Flash Translation Layer (FTL) readand write commands initiated as part of controller operations 640, suchas from FW scans, garbage collection, and the like, also result inrespective read and write operations among memory locations of thememory array 530. Further, system read level calibration commandsinitiated as part of the read level calibration 580, such as with thevoltage level calibration techniques discussed herein, result in variousread operations among memory locations of the memory array 530. A numberof other memory device management operations and commands are notillustrated in the diagram 600 for purposes of simplicity.

In an example, read operations and erase operations (e.g., eraseoperations implemented via write operations that re-write areas ofmemory) are tracked in connection with the read level calibration 580.The tracked read operations and erase operations may be utilized by theread level calibration 580 to detect that significant deviation from Vthas occurred, or is likely to occur, with a particular area of memory.For example, a block that has had repeated reads to it may be likely toencounter read disturb stresses. Likewise, a block that has been erasedrepeatedly is likely to encounter a higher level of wear, and thusexperience a higher fail bit count. As a result, the number of reads andthe number of erases occurring for a particular block or other area ofmemory may be tracked and used as a trigger, or as a contributing inputor condition, for performing voltage level adjustment operations withthe read level calibration 580.

In an example, a NAND erase counter 610 is used to track erase countsfor a particular memory location (or additionally, for multiple memorylocations) of the memory array 530. For instance, the erase counter 610may track erase operations at a block level, or at another level in thememory array 530 in which erase operations are implemented. In responseto an erase count for a respective block exceeding a determinedthreshold (ThresholdE) (determination 615), operations may be initiatedthat perform the read level calibration 580 on the respective block.

Likewise, in an example, a NAND read counter 620 is used to track readcounts for a particular memory location (or additionally, for multiplememory locations) of the memory array 530. For instance, the readcounter 620 may track read operations at a block level, or at anotherlevel in the memory array (e.g., per die) in which read operations areimplemented. In a further example, the read level calibration 580 isperformed per die, and thus the NAND block read counter 620 is used toaggregate a measurement of reads for all blocks within a respective die.In an example, the read counter 620 and the erase counter 610 are storedin DRAM (e.g., memory 544) of the storage device. In response to a readcount for a respective block or die exceeding a determined threshold(ThresholdR) (determination 625), operations may be initiated thatperform the read level calibration 580 on the respective block or die.

In an example, the read counter 620 is provided as a weight to the readlevel calibration 580, to identify whether a particular block or die islikely to experience voltage shift due to read usage (and thus, is morelikely to be in need of read voltage calibration). Also in an example,the erase counter 610 is used in combination with the read counter 620as a secondary metric for the read level calibration 580, to provide anadditional weight of whether a particular block is likely to experiencevoltage shift. However, in other examples, the read counter 620 and theerase counter 610 are used as thresholds for triggers that result involtage adjustment by the read level calibration 580.

In further examples, the tracking granularity or tracking frequency ofread or erase operations in the memory array 530 may be adjusted toreduce the overhead (e.g., processor and memory operations) used fortracking individual blocks or groups of blocks (dies). For instance,individual read operations may be tracked at a die level, rather than atan individual block or a page level; or, every x block in a group ofblocks, rather than every block, may be analyzed and tracked; or, everyn read operations, rather than each and every read operation, may beanalyzed and tracked.

In an example, the read level calibration 580 performs sampling of athreshold voltage of a number of identified locations in the memoryarray, in order to determine a voltage level for accessing theidentified locations (and, as applicable, other locations of the memoryarray). In this scenario, exceeding the erase threshold (e.g., fromdetermination 615) or exceeding the read threshold (e.g., fromdetermination 625) of a particular memory location might not directlytrigger or activate the read level calibration 580; instead, theparticular memory location may be added to a set of sampling locations650 used by the read level calibration 580, with the calibration beingtriggered or activated from another condition (e.g., a RBER rateexceeding a determined threshold). The tracking of the samplinglocations 650 may be implemented with any number of techniques,including those which prioritize certain memory locations based onfrequency, error rate, or the like.

In a further example, other locations in the memory array (e.g.,locations not identified as exceeding the read threshold (fromdetermination 625) or exceeding the erase threshold (from determination615)) may be identified and added to the set of sampling locations 650.These may include one or more locations determined based on one or moreof: raw bit error rate (RBER) of read operations occurring at aparticular location; data age; or random sampling.

In still further examples, the sampling operations that are performed bythe read level calibration 580 may allow configuration, such as from aspecification (e.g., a determined setting or calculation) of: a size ofdata (e.g., data corresponding to a page, block, group of blocks, die)that is sampled; a number of pages in total that are sampled; a numberof pages within a block that are sampled; whether certain cells, pages,blocks, dies, or certain types of such cells, pages, blocks, dies are orare not sampled; and the like. Likewise, the sampling that is performedby the read level calibration 580 may be adjusted according to certainbenchmarks, user patterns, read access patterns, or othercharacteristics to match the real-world actual or expected usage of thestorage device.

Also in a further example, the counters 610, 620 may decay or may resetas a result of managed operations within the controller (e.g.,controller operations 640). For example, folding of a particular blockwill result in garbage collection, consolidation, and erasure of theblock, resulting in an increment of the erase counter 610 and a reset ofthe read counter 620 for a particular block. The reset of the readcounter 620, in particular, may occur because the read disturb, dataretention, and cross-temperature effects are generally eliminated when ablock is erased and a memory cell returns to its native state. Othertechniques for emphasizing or de-emphasizing read or erases ofparticular memory locations involved with host IO may also be utilizedwith the counters 610, 620, the sampling locations 650, and the readlevel calibration 580.

FIG. 7 illustrates a flowchart 700 of an example set of operationsadapted for performing an optimized technique for read voltagecalibration. In an example, the operations of the flowchart 700 may beimplemented by a controller (e.g., controller 115, 540) of a storagedevice, through a combination of executed operations in software,firmware, or configured hardware. However, some or all aspects of thefollowing techniques may be implemented by other components (e.g., asinitiated by a host) in connection with other commands, controls, andforms of orchestration.

In an example, the operations of the flowchart 700 may be implemented ina memory device, the memory device comprising a NAND memory array havinggroups of multiple blocks of memory cells, and a memory controlleroperably coupled to the memory array, with the memory controller adapted(e.g., configured, arranged, programmed) to perform the respectiveoperations. In another example, the operations of the flowchart 700 maybe implemented in a method performed by or on a memory controller of aNAND memory array, the memory array having groups of multiple blocks ofmemory cells. In an example, the operations of the flowchart 700 may beimplemented in a device readable storage medium, which providesinstructions that perform the respective operations when executed (e.g.,when executed by a controller of a memory device).

The flowchart 700 is shown as commencing with the monitoring of IOoperations, such as read and erase operations occurring to a particularmemory address (operation 710). This monitoring is followed by anidentification of a particular location in the memory array (e.g., arespective block) that corresponds to the memory address (e.g., LBA) ofthe read or erase operation (operation 720). For example, the monitoringof the read and erase operations may determine whether a read hasoccurred to a respective block of the memory array, with this respectiveblock identified based on a block that includes a page corresponding tothe identified location (e.g., a location in the memory array that theLBA maps to).

The flowchart 700 continues with the tracking of counts of IO operationsoccurring to the specific location in memory (e.g., the respective blockor a group of blocks). As depicted, the tracking may include thetracking of read counts for a memory location with a read counter(operation 730) and the tracking of erase counts for a memory locationwith an erase counter (operation 740). These counts may be implementedin a read counter and erase counter, respectively, as indicated abovefor FIG. 6. For instance, the number of reads to the memory location maybe tracked in a first counter maintained in a memory accessible by thememory controller, and the number of erases to the memory location maybe tracked in a second counter maintained in the memory accessible bythe memory controller. In an example, the tracking of the read counts istracked at a respective block level, and the tracking of erase counts istracked at a respective block level. In another example, the tracking ofthe read counts is tracked at a die level (e.g., for a die including aplurality of blocks). Also in an example, an erase occurring to thememory location (e.g., an erase of the respective block) increments theerase counter while resetting the read counter (e.g., resetting tozero). In further examples, the erase counter is not implemented orutilized, allowing the tracking of the erase counts (in operation 740)to be an optional operation.

The flowchart 700 continues with the identification of a condition totrigger a read level calibration (operation 750). In an example, thecondition to trigger the read level calibration may be based on one ormore of: a read operation occurring to a respective block, a state of aread counter, or a state of an erase counter. As an example, thecondition to trigger the read level calibration may occur as a result ofmonitoring multiple read operations to a respective block of the memoryarray. In a further example, the condition is identified based on thenumber of reads occurring to the respective block exceeding a determinednumber of reads. Also as an example, the condition to trigger the readlevel calibration may be identified based on a number of erases to therespective block exceeding a determined number of erases.

In an additional example, the condition to trigger the read levelcalibration may occur based on the evaluation of groups of multipleblocks rather than a respective block, such as tracking a number ofreads occurring to a group of multiple blocks (the group hosting therespective block) to identify the condition when the reads exceed adetermined number of reads. Also in an additional example, the conditionto trigger the read level calibration may occur based in part on a rawbit error rate (RBER) of read operations occurring to at least therespective block of the memory array.

In response to the identified condition (from operation 750), a readlevel calibration operation may be performed on the memory device(operation 760). In an example, the read level voltage calibrationoperations include performing a sampling of various areas of a memoryarray, to sample for threshold voltage values and determine a readvoltage level. In an example, such sampling includes the sampling of athreshold voltage level of the respective block identified from themonitored read operations. In another example, such sampling includesthe sampling of a threshold voltage level of a respective group ofmultiple blocks hosting the respective block.

The read level calibration operation determines one or more values forread voltage that more accurately reflects the threshold voltage(s) forthe respective block; in turn, the read voltage level(s) used to readone or more page(s) of the respective block is updated with this value(operation 770). The updated read voltage level(s) then may be utilizedin subsequent read operations used to read one or more pages with therespective block (operation 780). In another example, the read voltagelevel(s) is updated and utilized on the basis of a group of blocks(e.g., for a respective die); in still another example, the read voltagelevel is tracked, updated, and utilized on the basis of a subset of therespective block (e.g., for a respective page or other set of memorycells within a block).

Based on the techniques discussed above, read voltage may be triggered,identified, and tracked based on the host IO state rather than anoverall, imprecise memory array state. Thus, it will be understood thatthe techniques discussed herein may be optimized to track the pattern ofreads occurring at a physical level, increasing the accuracy of anyresulting voltage adjustments. Further, the techniques for sampling andtriggering the voltage calibration may modified to be integrated withuse of existing calibration approaches, providing a minimal or even zeroimpact as compared with conventional sampling.

FIG. 8 illustrates a block diagram of an example machine 800 upon whichany one or more of the techniques (e.g., methodologies) discussed hereinmay perform. In alternative embodiments, the machine 800 may operate asa standalone device or may be connected (e.g., networked) to othermachines. In a networked deployment, the machine 800 may operate in thecapacity of a server machine, a client machine, or both in server-clientnetwork environments. In an example, the machine 800 may act as a peermachine in peer-to-peer (P2P) (or other distributed) networkenvironment. The machine 800 may be a personal computer (PC), a tabletPC, a set-top box (STB), a personal digital assistant (PDA), a mobiletelephone, a web appliance, an IoT device, automotive system, or anymachine capable of executing instructions (sequential or otherwise) thatspecify actions to be taken by that machine. Further, while only asingle machine is illustrated, the term “machine” shall also be taken toinclude any collection of machines that individually or jointly executea set (or multiple sets) of instructions to perform any one or more ofthe methodologies discussed herein, such as cloud computing, software asa service (SaaS), other computer cluster configurations.

Examples, as described herein, may include, or may operate by, logic,components, devices, packages, or mechanisms. Circuitry is a collection(e.g., set) of circuits implemented in tangible entities that includehardware (e.g., simple circuits, gates, logic, etc.). Circuitrymembership may be flexible over time and underlying hardwarevariability. Circuitries include members that may, alone or incombination, perform specific tasks when operating. In an example,hardware of the circuitry may be immutably designed to carry out aspecific operation (e.g., hardwired). In an example, the hardware of thecircuitry may include variably connected physical components (e.g.,execution units, transistors, simple circuits, etc.) including acomputer readable medium physically modified (e.g., magnetically,electrically, moveable placement of invariant massed particles, etc.) toencode instructions of the specific operation. In connecting thephysical components, the underlying electrical properties of a hardwareconstituent are changed, for example, from an insulator to a conductoror vice versa. The instructions enable participating hardware (e.g., theexecution units or a loading mechanism) to create members of thecircuitry in hardware via the variable connections to carry out portionsof the specific tasks when in operation. Accordingly, the computerreadable medium is communicatively coupled to the other components ofthe circuitry when the device is operating. In an example, any of thephysical components may be used in more than one member of more than onecircuitry. For example, under operation, execution units may be used ina first circuit of a first circuitry at one point in time and reused bya second circuit in the first circuitry, or by a third circuit in asecond circuitry at a different time.

The machine (e.g., computer system) 800 (e.g., the host device 105, thememory device 110, etc.) may include a hardware processor 802 (e.g., acentral processing unit (CPU), a graphics processing unit (GPU), ahardware processor core, or any combination thereof, such as the memorycontroller 115, etc.), a main memory 804 and a static memory 806, someor all of which may communicate with each other via an interlink (e.g.,bus) 808. The machine 800 may further include a display unit 810, analphanumeric input device 812 (e.g., a keyboard), and a user interface(UI) navigation device 814 (e.g., a mouse). In an example, the displayunit 810, input device 812 and UI navigation device 814 may be a touchscreen display. The machine 800 may additionally include a storagedevice (e.g., drive unit) 816, a signal generation device 818 (e.g., aspeaker), a network interface device 820, and one or more sensors 816,such as a global positioning system (GPS) sensor, compass,accelerometer, or other sensor. The machine 800 may include an outputcontroller 828, such as a serial (e.g., universal serial bus (USB),parallel, or other wired or wireless (e.g., infrared (IR), near fieldcommunication (NFC), etc.) connection to communicate or control one ormore peripheral devices (e.g., a printer, card reader, etc.).

The storage device 816 may include a machine readable medium 822 onwhich is stored one or more sets of data structures or instructions 824(e.g., software) embodying or utilized by any one or more of thetechniques or functions described herein. The instructions 824 may alsoreside, completely or at least partially, within the main memory 804,within static memory 806, or within the hardware processor 802 duringexecution thereof by the machine 800. In an example, one or anycombination of the hardware processor 802, the main memory 804, thestatic memory 806, or the storage device 816 may constitute the machinereadable medium 822.

While the machine readable medium 822 is illustrated as a single medium,the term “machine readable medium” may include a single medium ormultiple media (e.g., a centralized or distributed database, orassociated caches and servers) configured to store the one or moreinstructions 824.

The term “machine readable medium” may include any medium that iscapable of storing, encoding, or carrying instructions for execution bythe machine 800 and that cause the machine 800 to perform any one ormore of the techniques of the present disclosure, or that is capable ofstoring, encoding or carrying data structures used by or associated withsuch instructions. Non-limiting machine readable medium examples mayinclude solid-state memories, and optical and magnetic media. In anexample, a massed machine readable medium comprises a machine-readablemedium with a plurality of particles having invariant (e.g., rest) mass.Accordingly, massed machine-readable media are not transitorypropagating signals. Specific examples of massed machine readable mediamay include: non-volatile memory, such as semiconductor memory devices(e.g., Electrically Programmable Read-Only Memory (EPROM), ElectricallyErasable Programmable Read-Only Memory (EEPROM)) and flash memorydevices; magnetic disks, such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM and DVD-ROM disks.

The instructions 824 (e.g., software, programs, an operating system(OS), etc.) or other data are stored on the storage device 821, can beaccessed by the memory 804 for use by the processor 802. The memory 804(e.g., DRAM) is typically fast, but volatile, and thus a different typeof storage than the storage device 821 (e.g., an SSD), which is suitablefor long-term storage, including while in an “off” condition. Theinstructions 824 or data in use by a user or the machine 800 aretypically loaded in the memory 804 for use by the processor 802. Whenthe memory 804 is full, virtual space from the storage device 821 can beallocated to supplement the memory 804; however, because the storage 821device is typically slower than the memory 804, and write speeds aretypically at least twice as slow as read speeds, use of virtual memorycan greatly reduce user experience due to storage device latency (incontrast to the memory 804, e.g., DRAM). Further, use of the storagedevice 821 for virtual memory can greatly reduce the usable lifespan ofthe storage device 821.

In contrast to virtual memory, virtual memory compression (e.g., theLinux® kernel feature “ZRAM”) uses part of the memory as compressedblock storage to avoid paging to the storage device 821. Paging takesplace in the compressed block until it is necessary to write such datato the storage device 821. Virtual memory compression increases theusable size of memory 804, while reducing wear on the storage device821.

Storage devices optimized for mobile electronic devices, or mobilestorage, traditionally include MMC solid-state storage devices (e.g.,micro Secure Digital (microSD™) cards, etc.). MMC devices include anumber of parallel interfaces (e.g., an 8-bit parallel interface) with ahost device, and are often removable and separate components from thehost device. In contrast, eMMC™ devices are attached to a circuit boardand considered a component of the host device, with read speeds thatrival serial ATA™ (Serial AT (Advanced Technology) Attachment, or SATA)based SSD devices. However, demand for mobile device performancecontinues to increase, such as to fully enable virtual oraugmented-reality devices, utilize increasing networks speeds, etc. Inresponse to this demand, storage devices have shifted from parallel toserial communication interfaces. Universal Flash Storage (UFS) devices,including controllers and firmware, communicate with a host device usinga low-voltage differential signaling (LVDS) serial interface withdedicated read/write paths, further advancing greater read/write speeds.

The instructions 824 may further be transmitted or received over acommunications network 826 using a transmission medium via the networkinterface device 820 utilizing any one of a number of transfer protocols(e.g., frame relay, internet protocol (IP), transmission controlprotocol (TCP), user datagram protocol (UDP), hypertext transferprotocol (HTTP), etc.). Example communication networks may include alocal area network (LAN), a wide area network (WAN), a packet datanetwork (e.g., the Internet), mobile telephone networks (e.g., cellularnetworks), Plain Old Telephone (POTS) networks, and wireless datanetworks (e.g., Institute of Electrical and Electronics Engineers (IEEE)802.11 family of standards known as Wi-Fi®, IEEE 802.16 family ofstandards known as WiMax®), IEEE 802.15.4 family of standards,peer-to-peer (P2P) networks, among others. In an example, the networkinterface device 820 may include one or more physical jacks (e.g.,Ethernet, coaxial, or phone jacks) or one or more antennas to connect tothe communications network 826. In an example, the network interfacedevice 820 may include a plurality of antennas to wirelessly communicateusing at least one of single-input multiple-output (SIMO),multiple-input multiple-output (MIMO), or multiple-input single-output(MISO) techniques. The term “transmission medium” shall be taken toinclude any intangible medium that is capable of storing, encoding orcarrying instructions for execution by the machine 800, and includesdigital or analog communications signals or other intangible medium tofacilitate communication of such software.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples”. Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” may include “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the respectiveterms “comprising” and “wherein”. Also, in the following claims, theterms “including” and “comprising” are open-ended, that is, a system,device, article, or process that includes elements in addition to thoselisted after such a term in a claim are still deemed to fall within thescope of that claim. Moreover, in the following claims, the terms“first,” “second.” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements on their objects.

In various examples, the components, controllers, processors, units,engines, or tables described herein can include, among other things,physical circuitry or firmware stored on a physical device. As usedherein, “processor” means any type of computational circuit such as, butnot limited to, a microprocessor, a microcontroller, a graphicsprocessor, a digital signal processor (DSP), or any other type ofprocessor or processing circuit, including a group of processors ormulti-core devices.

The term “horizontal” as used in this document is defined as a planeparallel to the conventional plane or surface of a substrate, such asthat underlying a wafer or die, regardless of the actual orientation ofthe substrate at any point in time. The term “vertical” refers to adirection perpendicular to the horizontal as defined above.Prepositions, such as “on,” “over,” and “under” are defined with respectto the conventional plane or surface being on the top or exposed surfaceof the substrate, regardless of the orientation of the substrate; andwhile “on” is intended to suggest a direct contact of one structurerelative to another structure which it lies “on” (in the absence of anexpress indication to the contrary); the terms “over” and “under” areexpressly intended to identify a relative placement of structures (orlayers, features, etc.), which expressly includes—but is not limitedto—direct contact between the identified structures unless specificallyidentified as such. Similarly, the terms “over” and “under” are notlimited to horizontal orientations, as a structure may be “over” areferenced structure if it is, at some point in time, an outermostportion of the construction under discussion, even if such structureextends vertically relative to the referenced structure, rather than ina horizontal orientation.

The terms “wafer” and “substrate” are used herein to refer generally toany structure on which integrated circuits are formed, and also to suchstructures during various stages of integrated circuit fabrication. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the various embodiments is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

Various embodiments according to the present disclosure and describedherein include memory utilizing a vertical structure of memory cells(e.g., NAND strings of memory cells). As used herein, directionaladjectives will be taken relative a surface of a substrate upon whichthe memory cells are formed (i.e., a vertical structure will be taken asextending away from the substrate surface, a bottom end of the verticalstructure will be taken as the end nearest the substrate surface and atop end of the vertical structure will be taken as the end farthest fromthe substrate surface).

As used herein, directional adjectives, such as horizontal, vertical,normal, parallel, perpendicular, etc., can refer to relativeorientations, and are not intended to require strict adherence tospecific geometric properties, unless otherwise noted. For example, asused herein, a vertical structure need not be strictly perpendicular toa surface of a substrate, but may instead be generally perpendicular tothe surface of the substrate, and may form an acute angle with thesurface of the substrate (e.g., between 60 and 120 degrees, etc.).

In some embodiments described herein, different doping configurationsmay be applied to a source-side select gate (SOS), a control gate (CG),and a drain-side select gate (SGD), each of which, in this example, maybe formed of or at least include polysilicon, with the result such thatthese tiers (e.g., polysilicon, etc.) may have different etch rates whenexposed to an etching solution. For example, in a process of forming amonolithic pillar in a 3D semiconductor device, the SOS and the CG mayform recesses, while the SGD may remain less recessed or even notrecessed. These doping configurations may thus enable selective etchinginto the distinct tiers (e.g., SGS, CG, and SGD) in the 3D semiconductordevice by using an etching solution (e.g., tetramethylammonium hydroxide(TMCH)).

Operating a memory cell, as used herein, includes reading from, writingto, or erasing the memory cell. The operation of placing a memory cellin an intended state is referred to herein as “programming,” and caninclude both writing to or erasing from the memory cell (e.g., thememory cell may be programmed to an erased state).

According to one or more embodiments of the present disclosure, a memorycontroller (e.g., a processor, controller, firmware, etc.) locatedinternal or external to a memory device, is capable of determining(e.g., selecting, setting, adjusting, computing, changing, clearing,communicating, adapting, deriving, defining, utilizing, modifying,applying, etc.) a quantity of wear cycles, or a wear state (e.g.,recording wear cycles, counting operations of the memory device as theyoccur, tracking the operations of the memory device it initiates,evaluating the memory device characteristics corresponding to a wearstate, etc.)

According to one or more embodiments of the present disclosure, a memoryaccess device may be configured to provide wear cycle information to thememory device with each memory operation. The memory device controlcircuitry (e.g., control logic) may be programmed to compensate formemory device performance changes corresponding to the wear cycleinformation. The memory device may receive the wear cycle informationand determine one or more operating parameters (e.g., a value,characteristic) in response to the wear cycle information.

It will be understood that when an element is referred to as being “on,”“connected to” or “coupled with” another element, it can be directly on,connected, or coupled with the other element or intervening elements maybe present. In contrast, when an element is referred to as being“directly on,” “directly connected to” or “directly coupled with”another element, there are no intervening elements or layers present. Iftwo elements are shown in the drawings with a line connecting them, thetwo elements can be either be coupled, or directly coupled, unlessotherwise indicated.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, the code can be tangibly stored on one ormore volatile or non-volatile tangible computer-readable media, such asduring execution or at other times. Examples of these tangiblecomputer-readable media can include, but are not limited to, hard disks,removable magnetic disks, removable optical disks (e.g., compact discsand digital video disks), magnetic cassettes, memory cards or sticks,random access memories (RAMs), read only memories (ROMs), solid statedrives (SSDs), Universal Flash Storage (UFS) device, embedded MMC (eMMC)device, and the like.

Additional examples of the presently described embodiments are suggestedaccording to the structures and techniques described above and specifiedin the following examples and claims.

Example 1 is a memory device, comprising: a NAND memory array havinggroups of multiple blocks of memory cells; and a memory controlleroperably coupled to the memory array, the memory controller to performoperations comprising: monitoring read operations occurring to arespective block of the memory array; identifying, based at least inpart on the read operations occurring to the respective block, acondition to trigger a read level calibration; and performing, inresponse to the identified condition, the read level calibration basedon a threshold voltage level of a respective group of multiple blockshosting the respective block, wherein the read level calibration isperformed based at least in part on a threshold voltage to read therespective block.

In Example 2, the subject matter of Example 1 includes, monitoring theread operations by tracking a number of reads to the respective block,and wherein the condition to trigger the read level calibration isidentified, at least in part, based on the number of reads occurring tothe respective block exceeding a determined number of reads.

In Example 3, the subject matter of Example 2 includes, the operationsof the memory controller further comprising monitoring erase operationsoccurring to the respective block, wherein the condition to trigger theread level calibration is further identified based on a number of erasesto the respective block exceeding a determined number of erases.

In Example 4, the subject matter of Example 3 includes, the number ofreads to the respective block being tracked in a first countermaintained in a memory accessible by the memory controller, wherein thenumber of erases to the respective block is tracked in a second countermaintained in the memory accessible by the memory controller, andwherein an erase occurring to the respective block resets the firstcounter.

In Example 5, the subject matter of Examples 1-4 includes, monitoringthe read operations by tracking a number of reads to the respectivegroup of multiple blocks hosting the respective block, wherein thecondition to trigger the read level calibration is identified, at leastin part, based on the number of reads occurring to the respective groupof multiple blocks exceeding a determined number of reads.

In Example 6, the subject matter of Examples 1-5 includes, the conditionto trigger the read level calibration being further based in part on araw bit error rate (RBER) of read operations occurring to at least therespective block of the memory array.

In Example 7, the subject matter of Examples 1-6 includes, performingthe read level calibration by performing a sampling of the thresholdvoltage to read the respective block in addition to threshold voltagesto read other blocks located among the groups of multiple blocks in thememory array.

In Example 8, the subject matter of Example 7 includes, the other blocksin the memory array included in the sampling being identified within thememory array based on a number of read operations performed amongrespective blocks of the other blocks.

In Example 9, the subject matter of Examples 7-8 includes, the otherblocks in the memory array included in the sampling being identifiedwithin the memory array based on at least one of: random sampling of theother blocks in the memory array, sampling of the other blocks in thememory array based on data age, or sampling of the other blocks in thememory array based on raw bit error rate (RBER) corresponding to theother blocks.

In Example 10, the subject matter of Examples 1-9 includes, the readoperations to the respective block being monitored by monitoring readoperations to one or more portions of the respective block.

In Example 11, the subject matter of Example 10 includes, the monitoredportions of the respective block including pages of the respectiveblock.

In Example 12, the subject matter of Examples 1-11 includes, wherein thememory device is operably coupled to a host, wherein the host initiatescommands to perform respective reads among the multiple blocks in thememory array, and wherein the respective reads include multiple readsoccurring to a logical block address corresponding to a page locatedwithin the respective block.

In Example 13, the subject matter of Examples 1-12 includes, the readlevel calibration updating one or more read voltage levels used to readone or more pages of the respective block in subsequent read operationsoccurring to the one or more pages of the respective block.

In Example 14, the subject matter of Examples 1-13 includes, the blocksof memory cells of the memory array including at least one of:single-level cell (SLC), multi-layer cell (MLC), triple-layer cell(TLC), or quad-layer cell (QLC) NAND memory cells.

In Example 15, the subject matter of Examples 1-14 includes, the memoryarray being arranged into a stack of three-dimensional (3D) NAND dies,wherein the respective group of multiple blocks hosting the respectiveblock corresponds to a group of blocks provided by a respective die inthe stack of 3D NAND dies.

Example 16 is a method for optimizing voltage read level calibration ina memory device, the method comprising a plurality of operationsperformed by a memory controller of a NAND memory array, and the memoryarray having groups of multiple blocks of memory cells, with theoperations comprising: monitoring read commands issued to a respectiveblock of the memory array; identifying, based at least in part on theread commands issued to the respective block, a condition to trigger aread level calibration; and performing, in response to the identifiedcondition, the read level calibration based on a threshold voltage levelof a respective group of multiple blocks hosting the respective block,wherein the read level calibration is performed based at least in parton a threshold voltage to read the respective block; wherein the readlevel calibration updates a read voltage level used to read a page ofthe respective block in subsequent read operations to the page of therespective block.

In Example 17, the subject matter of Example 16 includes, monitoring theread operations by tracking a number of reads to the respective block,and wherein the condition to trigger the read level calibration isidentified, at least in part, based on the number of reads occurring tothe respective block exceeding a determined number of reads.

In Example 18, the subject matter of Example 17 includes, monitoringerase operations occurring to the respective block, wherein thecondition to trigger the read level calibration is further identifiedbased on a number of erases to the respective block exceeding adetermined number of erases.

In Example 19, the subject matter of Example 18 includes, the number ofreads to the respective block being tracked in a first countermaintained in a memory accessible by the memory controller, wherein thenumber of erases to the respective block is tracked in a second countermaintained in the memory accessible by the memory controller, andwherein an erase occurring to the respective block resets the firstcounter.

In Example 20, the subject matter of Examples 16-19 includes, monitoringthe read operations by tracking a number of reads to the respectivegroup of multiple blocks hosting the respective block, and wherein thecondition to trigger the read level calibration is identified, at leastin part, based on the number of reads occurring to the respective groupof multiple blocks exceeding a determined number of reads.

In Example 21, the subject matter of Examples 16-20 includes, thecondition to trigger the read level calibration being further based inpart on a raw bit error rate (RBER) of read operations occurring to atleast the respective block of the memory array.

In Example 22, the subject matter of Examples 16-21 includes, performingthe read level calibration by performing a sampling of the thresholdvoltage to read the respective block in addition to threshold voltagesto read other blocks located among the groups of multiple blocks in thememory array.

In Example 23, the subject matter of Example 22 includes, the otherblocks in the memory array included in the sampling being identifiedwithin the memory array based on a number of read operations performedamong respective blocks of the other blocks.

In Example 24, the subject matter of Examples 22-23 includes, the otherblocks in the memory array included in the sampling being identifiedwithin the memory array based on at least one of: random sampling of theother blocks in the memory array, sampling of the other blocks in thememory array based on data age, or sampling of the other blocks in thememory array based on raw bit error rate (RBER) corresponding to theother blocks.

In Example 25, the subject matter of Examples 16-24 includes, the readoperations to the respective block being monitored by monitoring readoperations to one or more portions of the respective block.

In Example 26, the subject matter of Example 25 includes, the monitoredportions of the respective block including pages of the respectiveblock.

In Example 27, the subject matter of Examples 16-26 includes, the memorydevice being operably coupled to a host, wherein the host initiatescommands to perform respective reads among the multiple blocks in thememory array, and wherein the respective reads include multiple readsoccurring to a logical block address corresponding to a page locatedwithin the respective block.

In Example 28, the subject matter of Examples 16-27 includes, the readlevel calibration updating one or more read voltage levels used to readone or more pages of the respective block in subsequent read operationsoccurring to the one or more pages of the respective block.

In Example 29, the subject matter of Examples 16-28 includes, the blocksof memory cells of the memory array including at least one of:single-level cell (SLC), multi-layer cell (MLC), triple-layer cell(TLC), or quad-layer cell (QLC) NAND memory cells.

In Example 30, the subject matter of Examples 16-29 includes, the memoryarray being arranged into a stack of three-dimensional (3D) NAND dies,wherein the respective group of multiple blocks hosting the respectiveblock corresponds to a group of blocks provided by a respective die inthe stack of 3D NAND dies.

Example 31 is a device readable storage medium, that providesinstructions that, when executed by a controller of a memory device,optimizes voltage read level calibration in the memory device, whereinthe instructions cause the controller to perform operations according toany of the techniques of Examples 1-30.

Example 32 is an apparatus comprising respective means for performingany of the methods or techniques of Examples 1-30.

Example 33 is a system, apparatus, or device to perform the operationsof any of Examples 1-30.

Example 34 is a tangible machine readable medium embodying instructionsto perform or implement the operations of any of Examples 1-30.

Example 35 is a method to perform the operations of any of Examples1-30.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment, and it is contemplated that such embodiments can be combinedwith each other in various combinations or permutations. The scope ofthe invention should be determined with reference to the appendedclaims, along with the full scope of equivalents to which such claimsare entitled.

What is claimed is:
 1. A memory device, comprising: a memory arraycomprising a group of multiple blocks of memory cells; and a memorycontroller operably coupled to the memory array, the memory controllerconfigured to perform operations comprising: identifying, based at leastin part on read operations to a block of the group, a condition totrigger a read level calibration; and performing, in response to theidentified condition, the read level calibration based on a thresholdvoltage level of the group and a threshold voltage to read the block. 2.The memory device of claim 1, wherein the operations further comprise:monitoring the read operations to the block.
 3. The memory device ofclaim 2, wherein the monitoring of the read operations includes trackinga number of reads to the block.
 4. The memory device of claim 3, whereinthe condition to trigger the read level calibration is identified, atleast in part, based on the number of reads to the block and apredetermined number of reads.
 5. The memory device of claim 2, whereinthe monitoring of the read operations to the block comprises monitoringread operations to the group comprising the block.
 6. The memory deviceof claim 1, wherein: the operations further comprise monitoring eraseoperations to the block; and the condition to trigger the read levelcalibration is identified, at least in part, based on a number of erasesto the block and a predetermined number of erases.
 7. The memory deviceof claim 6, wherein the operations further comprise: tracking a numberof reads to the block; and resetting the number of reads to the block inresponse to an erase of the block.
 8. The memory device of claim 1,wherein: the memory array comprises a second group of multiple blocks ofmemory cells, the second group comprising a second block; and theperforming of the read level calibration includes: sampling thethreshold voltage to read the block; and sampling a second thresholdvoltage to read the second block.
 9. The memory device of claim 8,wherein the second block is identified within the memory array based ona number of read operations to the second block.
 10. The memory deviceof claim 1, wherein the condition to trigger the read level calibrationis further based on a raw bit error rate (RBER) of read operations tothe block.
 11. The memory device of claim 8, wherein the second block isidentified within the memory array based on random sampling of theblocks of the memory array.
 12. The memory device of claim 8, whereinthe second block is identified within the memory array based on data ageof the blocks of the memory array.
 13. The memory device of claim 8,wherein the second block is identified within the memory array based onraw bit error rate (RBER) of the blocks of the memory array.
 14. Amethod comprising: identifying, based at least in part on readoperations to a block of a group of multiple blocks of memory cells in amemory array, a condition to trigger a read level calibration; andperforming, in response to the identified condition, the read levelcalibration based on a threshold voltage level of the group and athreshold voltage to read the block.
 15. The method of claim 14, furthercomprising: monitoring the read operations to the block.
 16. The methodof claim 15, wherein the monitoring of the read operations includestracking a number of reads to the block.
 17. The method of claim 16,wherein the condition to trigger the read level calibration isidentified, at least in part, based on the number of reads to the blockand a predetermined number of reads.
 18. A device readable storagemedium that stores instructions that, when executed by a controller of amemory device, cause the controller to perform operations comprising:identifying, based at least in part on read operations to a block of agroup of multiple blocks of memory cells in a memory array, a conditionto trigger a read level calibration; and performing, in response to theidentified condition, the read level calibration based on a thresholdvoltage level of the group and a threshold voltage to read the block.19. The device readable storage medium of claim 18, wherein: theoperations further comprise monitoring erase operations to the block;and the condition to trigger the read level calibration is identified,at least in part, based on a number of erases to the block and apredetermined number of erases.
 20. The device readable storage mediumof claim 18, wherein the operations further comprise: tracking a numberof reads to the block; and resetting the number of reads to the block inresponse to an erase of the block.